Solid-state lithium batteries were developed by Duracell in the 1970s and made commercially available in the 1980s, but are no longer produced. These cells included a lithium metal anode, a dispersed phase electrolyte of lithium iodide and Al2O3, and a metal salt as the cathode. The Li/LiI (Al2O3)/metal salt construction was a true solid-state battery, but these batteries were not rechargeable.
Because of the passivation reactions and unstable interfaces that form between organic electrolyte materials such as liquid and solid polymer electrolytes, it has long been a goal to develop a rechargeable solid state lithium based battery using an inorganic solid electrolyte material. In the early 1990s, a second type of all-solid-state battery was developed at the Oak Ridge National Laboratories. These cells consisted of thin films of cathode, inorganic electrolyte, and anode materials deposited on a ceramic substrate using vacuum deposition techniques, including RF sputtering for the cathode and electrolyte and vacuum evaporation of the Li metal anode. The total thicknesses of the cells were typically less than 10 μm: the cathode had a thickness of less than 4 μm, the solid electrolyte a thickness of around 2 μm (just sufficient to provide electrical isolation of the cathode and anode) and the Li anode a thickness of around 2 μm. Since strong chemical bonding (both within each layer and between the layers of the cell) was provided by the physical vapor deposition technique, the transport properties of these cells were excellent. Although the solid electrolyte LiPON has a conductivity of only 2×10−6 S/cm−1 (fifty times lower than that of the LiI(Al2O3) solid electrolyte used in the earlier Duracell battery), the impedance of the thin 2 μm layer was very small, allowing for very high rate capability. However, batteries based on this technology are very expensive to fabricate, are very small, and have very low capacity.
Solid-state batteries are the focus of a great deal of attention because of the potential for attractive performance properties, including long shelf life, long term stable power capability, no gassing, broad operating temperature range (−40° C. to 170° C. for pure lithium anodes and up to and beyond 300° C. using active composite anodes), and high volumetric energy density (up to 2000 Wh/L). They are particularly suited for applications requiring long life under low-drain or open-circuit conditions.
Currently, Li-ion battery chemistry using liquid electrolyte provides the best known performance and is becoming the most widely used of all battery chemistries. Lithium ion cells consist of thick (˜100 μm) porous composite cathodes cast on a thin (˜10 μm) Al foil current collector. The composite cathode typically contains both LiCoO2 as the active material, due to its high capacity and good cycle life, and carbon black, which provides electrical conductivity throughout the layer. A number of active cathode materials have been and are being investigated in an effort to improve battery performance. Some of these materials have been implemented in cells, including Lithium Nickel Cobalt Manganese Oxide. A thin polymer separator provides electrical isolation between the cathode and the carbon-based anode. The anode intercalates Li during the charge cycle. The cell is immersed in a liquid electrolyte, which provides very high conductivity for the transport of Li ions between the cathode and anode during charge and discharge. Because the separator and composite cathode and anode are all porous, the liquid electrolyte is absorbed into and fills the structure, thus providing excellent surface contact with the LiCoO2 active material and allowing fast transport of Li ions throughout the cell with minimal impedance.
The liquid electrolyte itself consists of a Li salt (for example, LiPF6) in a solvent blend which typically includes ethylene carbonate and other linear carbonates, such as dimethyl carbonate. Despite improvements in energy density and cycle life, there remain several underlying problems with batteries that contain liquid electrolytes. For example, liquid electrolytes are generally volatile and subject to pressure build up, explosion, and fire under a high charge rate, a high discharge rate, and/or internal short circuit conditions. Additionally, charging at a high rate can cause dendritic lithium growth on the surface of the anode. The resulting dendrites can extend through the separator and internally short circuit in the cell. Further, the self-discharge and efficiency of the cell is limited by side reactions and corrosion of the cathode by the liquid electrolyte. Still further, the liquid electrolyte also creates a hazard if the cell over-heats due to overvoltage or short circuit conditions, creating another potential fire or explosion hazard.
To address safety and reliability problems with lithium based batteries that employ liquid electrolytes, and to achieve high energy density, solid-state batteries that employ high capacity lithium intercalation compounds are being developed. Past attempts at constructing such all-solid-state batteries have been limited by the need to bind the materials together in order to facilitate effective lithium ion transport across interfaces. This binding process has been attempted by sintering at high temperature, such as 800° C. and higher. However, the cathode and electrolyte materials may react with each other at such sintering temperatures, resulting in high impedance interfaces and an ineffective battery.
To avoid the parasitic reaction problems associated with high temperature sintering, all solid state batteries have been developed using a low temperature sol gel process. These all-solid-state batteries consist of a composite cathode containing active battery cathode material (e.g., LiNiMnCoO2, LiCoO2, LiMn2O4, Li4Ti5O12 or similar), an electrically conductive material (e.g., carbon black), and lithium ion conductive glass electrolyte material, such as Li3xLa2/3-xTiO3 (x=0.11) (LLTO) or Li7La3Zr2O12 (LLZO) that may be formed in situ from a liquid, organic precursor. When gelled and subsequently cured at low temperature, the precursor is transformed into a solid lithium ion conductive glass electrolyte.
In constructing a solid-state battery using the low temperature sol gel approach, a cathode may be formed by mixing a lithium active material, an electrically conductive material, and the liquid sol gel precursor to form a homogenous mixture or paste. The cathode may be formed as either a thick pellet or as a thin casting containing the mixture of cathode components. The cathode is held together by the ion conductive glass electrolyte matrix that is formed by gelling and curing the sol-gel precursor solution. Curing temperature for the gelled precursor is in the range of 300° C., thus avoiding parasitic reactions.
However, construction of battery electrodes using the sol gel approach to produce glass electrolyte as a binder requires proper gelling, drying, and curing of the precursor. Gelling of precursors for LLTO and LLZO is a hygroscopic process. Moisture must diffuse into the cathode structure through the tortuous path formed by the densely packed cathode powder materials in order for the cathode material to gel properly throughout. Drying of the precursor after gelling may be time consuming because solvents and alcohols must diffuse through the gelled electrolyte within the tortuous compacted electrode powder structure.
The all-solid-state primary cell developed by Duracell and described above demonstrated very high energy densities of up to 1000 Wh/L and excellent performance in terms of safety, stability, and low self-discharge. However, due to the pressed powder construction and the requirement for a thick electrolyte separation layer, the cell impedance was very high, severely limiting the discharge rate of the battery. This type of cell is also restricted in application because the electrochemical window is limited to less than three volts due to the iodide ions in the electrolyte, which are oxidized above approximately three volts. In addition, a stable rechargeable version of this cell was never developed.
The all-solid-state thin film battery developed by Oak Ridge National Laboratories, also detailed above, addresses many of the problems associated with Li− ion technology, but also has limitations. The vacuum deposition equipment required to fabricate the cells is very expensive and the deposition rates are slow, leading to very high manufacturing costs. Also, in order to take advantage of the high energy density and power density afforded by use of the thin films, it is necessary to deposit the films on a substrate that is much thinner and lighter than the battery layers themselves so that the battery layers make up a significant portion of the volume and weight of the battery compared to the inert substrate and packaging components. Ideally, one would simply use thicker battery electrode layers and thereby make the substrate a less significant percentage of the battery's volume; however, it is not practical to increase the electrode thickness beyond a few microns. Low lithium diffusion rates coupled with thick electrode layers result in an impractical battery with low charge and discharge rates. Therefore the films must be deposited on very thin substrates (<10 μm) or multiple batteries must be built up on a single substrate, which leads to problems with maintaining low interface impedance with the electrolyte during the required high temperature annealing of the cathode material after deposition.
Metal oxide electrolytes having conductivities in the range of 10−3 S/cm have been fabricated. However, the use of such materials as solid electrolytes in all-solid-state batteries has been limited, in part due to the high interface impedance that results from the high temperature sintering process used to form bonds between the electrolyte and active cathode materials. While bonding is needed to enable lithium ion conduction between the materials, inter-atomic migration during sintering results in very high interface impedance and very limited functionality of resulting cells.
Even though solid state batteries have been made by homogenous mixtures of electrolyte and active material powders and bonded together using low temperature processing to yield low interface impedance, improved charge/discharge rate capability, and access to the full capacity of thicker cathodes has remained very limited. FIG. 1 shows the various layers of a solid state cell, including cathode current collector 8, cathode 6, electrolyte separator 4, and anode layer 2 constructed using the prior state of the art approach. In the expanded view, solid electrolyte particles 12 are shown embedded within cathode active material 10.
Cathode 6 is constructed having enough solid electrolyte material 12 to achieve percolation such that there is a network of particles in contact with each other to achieve ionic conduction continuity. The standard construction procedure for the cathode is to mix the constituent cathode powder materials until the electrolyte particles are relatively homogenously distributed. The relatively uniform, but random, distribution is maintained during construction of the battery cell such that the configuration shown in FIG. 1 is representative of a completed battery in accordance with the prior art. It illustrates some of challenges faced with constructing solid state cells, particularly those with relatively thick cathodes. Because of the random mixing process, some percentage of the electrolyte material, particles and group of particles, will naturally be surrounded by active cathode material and thereby isolated from the electrolyte network, as illustrated by particles 14. These isolated particles cannot participate in transporting lithium ions into the cathode from the separator. For example, consider lithium ion 22 conducted through electrolyte layer 4. It continues a conductive path through electrolyte particle 24. It receives an electron 18 or 20 and transitions into active material 10. In receiving an electron, it returns to its full lithium state 26 and intercalates into the cathode material 10 by diffusion. Once in a full lithium state, the atom cannot enter an electrolyte particle 25, release an electro 28, and have that electron conduct via a parallel path 28 so that it is reconstituted as lithium at 30, thereby conducting deep within the cathode. Once intercalated as lithium into the active cathode material, its transport within the cathode will be by diffusion, which is too slow for most applications.
Another problem is the limited cross sectional area where electrolyte particles connect to each other, as represented at 15. These areas of limited interface are like conduction choke points. They tend to cause increased impedance due to the small contact areas between particles.
Still another problem is represented by the network of particles 16. Ideally lithium ion 17 enters the network and is conducted through a series of interconnected particles to be intercalated into active material 10 at location 19. This is a tortuous path that is worsened by the fact that the ion must be conducted in a direction opposite to that of the electronic charge field to be intercalated at 19. It is not clear that this would occur, given the positive charge of the lithium ion.
The net effect is that solid state batteries with cathodes having a random distribution of conductive electrolyte and active particles display limited performance. Therefore, the need remains for a solid state cell structure and production process which provides high rate capability and effective transport of lithium within the structure of the resident electrode.